The sources of noise in a vehicle are many and include, among others, power train, driveline, tire contact patch (excited by the road surface), brakes, and wind. The noise generated by all these sources inside the vehicle's cabin covers a rather large frequency range that, for normal diesel and petrol vehicles, can be as high as 6.3 kHz (above this frequency, the acoustical power radiated by the noise sources in a vehicle is generally negligible). Vehicle noise is generally divided into low, middle and high frequency noise. Typically, low frequency noise can be considered to cover the frequency range between 50 Hz and 500 Hz and is dominated by “structure-borne” noise: vibration is transmitted to the panels surrounding the passengers' cabin via a variety of structural paths and such panels then radiate noise into the cabin itself. On the other hand, typically high-frequency noise can be considered to cover the frequency range above 2 kHz. High-frequency noise is typically dominated by “airborne” noise: in this case the transmission of vibration to the panels surrounding the passengers' cabin takes place through airborne paths. It is recognized that a grey area exists, where the two effects are combined and neither of the two dominates. However, for passenger comfort, it is important that the noise is attenuated in the middle frequency range as well as in the low and high frequency ranges.
For noise attenuation in vehicles, including cars and trucks, it is well known to use insulators, dampers, and absorbers to reflect and dissipate sound, and thus reduce the overall interior sound level.
Insulation is traditionally obtained by means of a “mass-spring” barrier system including a mass element and a spring element. The mass element is formed from a layer of high density impervious material, normally designated as a heavy layer, and the spring element is formed from a layer of low density material, for example a non-compressed felt or foam.
The term “mass-spring” is commonly used to define a barrier system that provides sound insulation through the combination of two elements, the “mass” and “spring”. A part or a device is said to work as a “mass-spring” if its physical behaviour can be represented by the combination of a mass element and a spring element. These mechanical components of the mass-spring system, which are bonded together, allow the system to act as a sound insulator.
A traditional mass-spring system is normally put in a car on top of a steel layer, with the spring element in contact with the steel. The complete system (mass-spring plus steel layer) has the characteristics of a double partition. An insertion loss quantity describes how effective the mass-spring system is when put on top of the steel layer, independently from the insulation provided by steel layer itself. The insertion loss therefore shows the insulation performance of the mass-spring system.
A theoretical insertion loss curve (IL, measured in dB) includes particular features that characterize a mass-spring system. With regard to frequency, the curve increases with the frequency in an approximately linear fashion, and the rate of growth is about 12 dB/octave. This linear trend is considered very effective to guarantee a good insulation against incoming sound waves and, for this reason, mass-spring systems have been widely used in the automotive industry. This trend is achieved only above a certain frequency value, called “resonance frequency of the mass-spring system”, at which the system is not effective as a sound insulator. The resonance frequency depends mainly on the weight of the mass element (the higher the weight, the lower the resonance frequency) and on the stiffness of the spring (the higher the stiffness, the higher the resonance frequency). At the resonance frequency of the mass-spring system, the spring element transmits the vibration of the underlying structure to the mass element in a very efficient way. At this frequency, the vibration of the mass element is even higher than that of the underlying structure, and thus the noise radiated by the mass element is even higher than the one that would be radiated by the underlying structure without the mass-spring system. As a consequence, around the resonance frequency of the mass-spring system, the IL curve has a negative minimum.
The insulation performance of an acoustical barrier is assessed by sound transmission loss (TL). The ability of an acoustical barrier to reduce the intensity of the noise being transmitted depends on the nature of the materials forming the barrier. An important physical property controlling sound TL of an acoustical barrier is the mass per unit area of its component layers. For best insulating performance, the heavy layer of a mass-spring system will often have a smooth high-density surface to maximize reflection of noise waves, a non-porous structure, and comprise a material of sufficient stiffness to minimize vibration.
Typical classical mass layers are made of highly filled dense materials, such as EPDM, EVA, PU, PP etc. These materials have a high density, normally above 1000 (kg/m3), a smooth surface to maximize reflection of noise waves, a non-porous structure, and a certain stiffness to minimize vibration. It is known that many textile fabrics, either thin and/or porous in structure, are not ideal for noise insulation.
Absorption is usually obtained by the use of porous layers. The absorbing performance of an acoustical system is assessed by the absorption coefficient (a dimensionless quantity). Absorbers are commonly made of open porous materials, for example felt or foams.
Both absorbing and insulating systems work optimally only within a small bandwidth of frequencies. An absorber generally works better in higher frequencies, while an insulator generally works better in lower frequencies. Furthermore, both systems are sub optimal for use in a modern vehicle. The effectiveness of an insulator is strongly dependent on its weight, the higher the weight the more effective the insulator. The effectiveness of an absorber, on the other hand, is strongly dependent on the thickness of the material, the thicker the better. Both thickness and weight are becoming increasingly restricted, however. For example, additional weight of the insulator negatively impacts the vehicle's fuel economy, and the additional thickness of the absorber material-affects the vehicle's spaciousness.
Traditional insulators of the mass-spring type typically include a mass layer that is not porous, and therefore have low absorption, close to zero. The mass spring system only shows a noticeable absorption peak in a narrow band around the resonance frequency. However the absorption peak is in the low frequency region and not in the area of interest for absorption, which is the middle and high frequency region.
In the past, many attempts have been made at optimizing the sound insulation in a vehicle in such a way to reduce its mass (weight) while keeping the same level of acoustic comfort. The-potential for such a weight optimization is mainly-found in the heavy layer, and therefore, the optimization attempts have concentrated on reducing the mass of the heavy layer. However, these attempts have shown that if the weight of the heavy layer is reduced beyond a certain physical limit, the insulation system does not behave as a mass-spring system any longer and a loss of acoustic comfort inevitably occurs. In recent years, additional absorbing material was added to compensate for this loss of acoustic comfort.
Traditionally, one method to reduce the weight of the heavy layer included using fully porous systems. However porous absorbers have a very low acoustic insulation. For a porous system, the IL curve increases with the frequency in an approximately linear way, but only with a growth rate of about 6 dB/Octave instead of the 12 dB/Octave that can be observed when using an impervious barrier material like a heavy layer.
Another common practice for dealing with the above mentioned problem has consisted in putting an absorbing material on top of a mass spring system. With such a configuration, it is expected that the presence of the additional material would mainly add absorbing properties to the sound attenuating system. Additionally, it is also expected that the additional material would positively impact the acoustic insulation of the underlying mass-spring system since the material increases the overall weight of the system.
Products of this type are often referred to as ABA (Absorber-Barrier-Absorber) systems. Most of the ABA systems are made with foam or felt as a first absorbing layer, a barrier for example in the form of a heavy layer material as discussed, and an absorbing layer that functions also as a spring layer for the mass spring system. Also this absorbing layer typically includes a felt or foam. The barrier layer, together with the absorbing layer directly in contact with the structure on which the system is applied, should function as a mass spring system. The top absorbing layer should function as an additional sound absorber.
It is expected that when additional weight is put on top of a mass-spring system, such additional weight should affect the insulation performance of the system positively; for instance, an addition of 250 (g/m2) of material on top of a mass spring system with a 2 (kg/m2) heavy layer should give an overall IL increase of approximately 1 (dB), while an addition of 500 (g/m2) of material on top of the same system should already give an IL increase of 2 dB. An IL increase of more than 1 dB is normally considered relevant for the overall noise attenuation in the passenger compartment of a vehicle. For a (kg/m2) heavy layer, already an addition of 150 (g/m2) of material should give such a 1 (dB) effect.
Surprisingly, it was found that when an absorbing layer is added on top of a mass-spring system to obtain an ABA system with a heavy layer as a barrier, the increase in the system's IL is much lower than what would be expected from the added weight. In many cases, the addition of the absorbing layer leads even to a reduction of the system's IL.
Many applications of ABA systems use a very soft felt (commonly designated as “fleece”), with an area weight between 400 and 600 g/m2, as an absorbing top layer. This felt absorber is mechanically very soft (its compression Young's modulus is very low, typically much lower than the one of standard air), and does not participate actively in the insulating function of the system. Specifically, the link between the fibres of the absorber and the underlying heavy layer is not strong enough to affect the mass of the system. As a result, the addition of the absorber does not lead to any increase in the system's IL, and the system's insulation function is determined only by the mass of the heavy layer that is put on top of the decoupling layer. Very soft felt materials (or “fleeces”) are more expensive than common thermo formable fibrous materials and therefore are normally only applied in the form of patches on top of the mass-spring system. Such application has to be accomplished manually and this can be an expensive operation.
As an alternative, the ABA system can be obtained by moulding or gluing a more traditional thermo-formable felt with, for example, an area weight between 500 and 2000 (g/m2) on top of the heavy layer, to act as an absorber. Unexpectedly, it was found that this top absorbing layer has a negative effect on the insulation performance of the underlying mass-spring system, producing a deterioration of its IL curve. Such deterioration is caused by the noise radiation of the system formed by the heavy layer and the absorbing top layer. In fact, a specific frequency exists, radiation frequency, at which vibrations are transmitted by the heavy layer to the top absorbing layer in a very efficient way, thus making the top absorbing layer radiate noise. At the radiation frequency, the top surface of the top absorbing layer vibrates more than the underlying heavy layer. Due to this effect, the insertion loss of the ABA system is strongly compromised in the frequency range around the radiation frequency. In this frequency range, the IL of the ABA system is lower than the IL of the mass-spring system from which it is obtained. In this way, the addition of an acoustic function (absorption, via the absorber added on top) significantly deteriorates the original function of the system, i.e. insulation. The acoustic radiation of the system formed by the heavy layer and the top porous layer together deteriorates the insulation of the system, a case that was not considered previously in the state of the art.